Dispersion spun or wet PTFE yarns are typically produced by forming a spin mix containing an aqueous dispersion of poly(tetrafluoroethylene) particles and a solution of a cellulosic ether matrix polymer. The spin mix is then extruded at relatively low pressure (e.g., less than 150 pounds per square inch) through an orifice into a coagulation solution usually containing sulfuric acid to coagulate the matrix polymer and form an intermediate fiber structure. The intermediate fiber structure, once washed free of acid and salts, is passed over a series of heated rolls to dry the fiber structure and sinter the PTFE particles into a continuous PTFE filament yarn.
In order to increase PTFE yarn productivity and improve the yarn's functional properties (e.g., tenacity), the dried and sintered yarn is often drawn by accelerating the yarn speed over the last pair of heated rolls by passing the yarn onto a series of draw rolls having a rotational speed greater than the rotational speed of the heated rolls. Thus, the yarn is drawn or stretched over the last pair of heated rolls since it is being retrieved by the drawing rolls faster than it is being supplied by the heated rolls. The amount the yarn is drawn is referred to as the draw length or draw ratio. Typical draw ratios for a dispersion spun PTFE yarn range between 6.7 and 7.4, (i.e., the yarn is drawn to a length that is between 6.7 and 7.4 times greater than its pre-drawn length). After drawing, the yarn is wound into packages.
Although drawing PTFE yarn increases the tenacity of the yarn, it has the undesired effect of decreasing the yarn's thermal stability and elongation prior to break of the yarn. Accordingly, what is needed is a method of making a dispersion spun PTFE yarn that allows for increased productivity while maintaining or increasing yarn thermal stability and elongation prior to break of the yarn.
The primary benefit of maintaining or increasing yarn thermal stability in a dispersion spun PTFE yarn is centered in the hot gas filtration market. Because filter media made from PTFE yarn are exposed to and in continuous service in applications where air temperatures are regularly at or above 260 degrees Celsius, it is necessary to heat treat the PTFE yarn prior to putting it into service. When this step is accomplished standard yarns produced by dispersion spinning PTFE homopolymer shrink 20% or more. While the resulting shrunken PTFE yarn filter media performs well, it requires users to buy greater amounts of PTFE yarn to cover the loss of filter surface area caused by the shrinking.